Surgical cutting elements consisting of a specially formed thermistor as the core of the element are already known. These cutting instruments, which simultaneously serve as thermal sensors and as heating elements, are linked to a bridge connection (see German Offenlegungsschrift No. 2,423,537). The bridge is operated by an alternating current voltage of at least 24 Vrms. The circuit works as follows: a change in the operating temperature on the cutting instrument brings about a variation in the bridge. This causes the control signal in the diagonals of the bridge to be either in phase or out of phase with the alternating current voltage supplied. With this control signal the power is varied via a thyristor circuit, depending on the state of the bridge. The instrument is consequently able to deliver relatively quickly in a short time the heat given off. This known circuit has, however, two substantial disadvantages which do not permit such a circuit to be used universally, especially for precisely working surgical instruments: According to current regulations, medical instruments which come in direct contact with the body of the patient may not be operated at a voltage which is higher than 24 V. On the known device the rms voltage (Vrms) is 24 V. The peak voltage is obtained at 67.68 V. A lowering of this alternating-current voltage to the permissible voltage of 8.5 Vrms corresponding to 24 V peak-to-peak would mean that it would no longer be possible to furnish the required power for the cutting instrument. With the above-described type of circuit it is not possible to construct an appliance meeting the safety requirements for medical instruments.
Under the above-mentioned conditions, and especially also in view of the large passive thermal resistances of the cutting instrument, the readjustment time would become far too long at the reduced power. The effect intended with the circuit of maximal thermal constancy and rapid furnishing of heat would no longer be possible. This cutting instrument can therefore be operated only under conditions which no longer meet the safety regulations for medical instruments.